1. Field of the Invention
The present invention relates to an improved microwave-interactive cooking package. In particular, the present invention relates to high efficiency, safe and abuse-tolerant susceptor and foil materials for packaging and cooking microwavable food.
2. Description of the Related Art
Although microwave ovens have become extremely popular, they are still seen as having less than ideal cooking characteristics. For example, food cooked in a microwave oven generally does not exhibit the texture, browning, or crispness that are acquired when food is cooked in a conventional oven.
A good deal of work has been done in creating materials or utensils that permit food to be cooked in a microwave oven to obtain cooking results similar to that of conventional ovens. The most popular device being used at present is a plain susceptor material, which is an extremely thin (generally 60 to 100 xc3x85) metallized film that heats under the influence of a microwave field. Various plain susceptors (typically aluminum, but many variants exist) and various patterned susceptors (including square matrix, xe2x80x9cshower flowerxe2x80x9d, hexagonal, slot matrix and xe2x80x9cfusexe2x80x9d structures) are generally safe for microwave cooking. However, susceptors do not have a strong ability to modify a non-uniform microwave heating pattern in food through shielding and redistributing microwave power. The quasi-continuous electrical nature of these materials prevents large induced currents (so limiting their power reflection capabilities) or high electromagnetic (E-field) strengths along their boundaries or edges. Therefore their ability to obtain uniform cooking results in a microwave oven is quite limited.
Electrically xe2x80x9cthickxe2x80x9d metallic materials (e.g., foil materials) have also been used for enhancing the shielding and heating of food cooked in a microwave oven. Foil materials are much thicker layers of metal than the thin, metallized films of susceptors. Foil materials, also often aluminum, are quite effective in the prevention of local overheating or hot spots in food cooked in a microwave by redistributing the heating effect and creating surface browning and crisping in the food cooked with microwave energy. However, many designs fail to meet the normal consumer safety requirements by either causing fires, or creating arcing as a result of improper design or misuse of the material.
The reason for such safety problems is that any bulk metallic substance can carry very high induced electric currents in opposition to an applied high electromagnetic field under microwave oven cooking. This results in the potential for very high induced electromagnetic field strengths across any current discontinuity (e.g., across open circuit joints or between the package and the wall of the oven). The larger the size of the bulk metallic materials used in the package, the higher the potential induced current and induced voltage generated along the periphery of the metallic substance metal. The applied E-field strength in a domestic microwave oven might be as high as 15 kV/m under no load or light load operation. The threat of voltage breakdown in the substrates of food packages as well as the threat of overheating due to localized high current density may cause various safety failures. These concerns limit the commercialization of bulk foil materials in food packaging.
Commonly owned Canadian Patent No. 2196154 offers a means of avoiding abuse risks with aluminum foil patterns. The structure disclosed addresses the problems associated with bulk foil materials by reducing the physical size of each metallic element in the material. Neither voltage breakdown, nor current overheat will occur with this structure in most microwave ovens, even under abuse cooking conditions. Abuse cooking conditions can include any use of a material contrary to its intended purpose including cooking with cut or folded material, or cooking without the intended food load on the material. In addition, the heating effectiveness of these metallic materials is maximized through dielectric loading of the gaps between each small element that causes the foil pattern to act as a resonant loop (albeit at a much lower Q-factor (quality factor) than the solid loop). These foil patterns were effective for surface heating. However, it was not recognized that a properly designed metallic strip pattern could also act to effectively shield microwave energy to further promote uniform cooking.
Commonly owned U.S. Pat. No. 6,133,560 approaches the problem differently by creating low Q-factor resonant circuits by patterning a susceptor substrate. The low Q-factor operation described in U.S. Pat. No. 6,133,560 provides only a limited degree of power balancing.
The present invention relates to an abuse-tolerant microwave packaging material which both shields food from microwave energy to control the occurrence of localized overheating in food cooked in a microwave, and focuses microwave energy to an adjacent food surface.
Abuse-tolerant packaging according to the present invention includes one or more sets of continuously repeated microwave-interactive metallic segments disposed on a microwave-safe substrate. Each set of metallic segments defines a perimeter equal to a predetermined fraction of the effective wavelength in an operating microwave oven. Methodologies for choosing such predetermined fractional wavelengths are discussed in U.S. Pat. No. 5,910,268, which is incorporated herein by reference. The metallic segments can be foil segments, or may be segments of a high optical density evaporated material deposited on the substrate. The terms xe2x80x9cfractionxe2x80x9d or xe2x80x9cfractionalxe2x80x9d as used herein are meant in their broadest sense as the numerical representation of the quotient of two numbers, i.e., the terms include values of greater than, equal to, and less than one (1).
In a first embodiment, the length of the perimeter defined by a first set of metallic segments is preferably approximately equal to an integer multiple of the effective wavelength of microwaves in an operating microwave oven, such that the length of the perimeter is resonant with the effective wavelength. In a second embodiment, the length of the perimeter defined by the metallic segments is approximately equal to an integer multiple of one-half the effective wavelength of microwaves in an operating microwave oven, such that the length of the second perimeter is quasi-resonant with the effective wavelength.
Each segment in the first set is spaced from adjacent segments so as to create a (DC) electrical discontinuity between the segments. Preferably, each first set of metallic segments defines a five-lobed flower shape. The five-lobed flower shape promotes uniform distribution of microwave energy to adjacent food by distributing energy from its perimeter to its center.
Preferably, abuse-tolerant packaging according to the present invention includes a repeated second set of spaced metallic segments that enclose each first set of metallic segments and define a second perimeter. In the first embodiment, this second perimeter preferably has a length approximately equal to an integer multiple of the effective wavelength of microwaves in an operating microwave oven, such that the length of the second perimeter is resonant with the effective wavelength. In the second embodiment, this second perimeter preferably has a length approximately equal to an integer multiple of one-half the effective wavelength of microwaves in an operating microwave oven, such that the length of the second perimeter is quasi-resonant with the effective wavelength.
A third embodiment of abuse-tolerant packaging according to the present invention includes, in addition to the second set of metallic segments, a repeated third set of spaced metallic segments that enclose each second set of metallic segments and define a perimeter approximately equal to another predetermined fraction of the effective wavelength of microwaves in an operating microwave oven.